Among publications on the subject of the preparation of propylene oxide, there are only a few which are concerned with the treatment of the waste water and recovering byproducts from the reaction mixture.
WO 2004/000773 refers to a process of separating 1-methoxy-2-propanol and 2-methoxy-1-propanol from aqueous compositions, comprising: dewatering of the aqueous composition comprising 1-methoxy-2-propanol and 2-methoxy-1-propanol to a concentration of 1-methoxy-2-propanol and 2-methoxy-1-propanol of at least 90 percent by weight in total and isolation of 1-methoxy-2-propanol, 2-methoxy-1-propanol or mixtures thereof by means of distillation. According to WO 2004/000773, dewatering of the aqueous composition can be achieved by a membrane separation. Thereby the aqueous composition comprising 2-methoxy-1-propanol and 1-methoxy-2-propanol is contacted with a semipermeable, hydrophilic membrane in a suitable apparatus either in liquid phase as pervaporation step or in the gaseous phase as vapor permeation step. Across the semipermeable membrane a pressure difference is established. The permeate will substantially contain water and only a minor amount of 1-methoxy-2-propanol and 2-methoxy-1-propanol. According to WO 2004/000773, the major amount of 1-methoxy-2-propanol and 2-methoxy-1-propanol fed to the apparatus will not pass the membrane and can be collected as a retentate with a reduced water content.
U.S. Pat. No. 5,599,955 provides an integrated process for the production of propylene oxide from an alternate feedstream such as synthesis gas. In the process, propylene oxide is produced from a feedstream comprising hydrogen and a carbon oxide. The propylene stream is epoxidized with hydrogen peroxide which has been produced from hydrogen separated from a portion of the feedstream. The spent water stream produced by the epoxidation reaction is treated to remove heavy components and returned to the hydrogen peroxide production zone. The recycling of spent water from the epoxidation reaction zone and the removal of heavy compounds eliminates a low value water stream and the recovery of heavy hydrocarbons therefrom produces a valuable secondary product. The spent water stream is passed to a separation zone such as an evaporator, a distillation zone or a sorption zone.
US 2002/010378 discloses a composite process for subjecting ethylene to catalytic gas phase oxidation thereby obtaining ethylene oxide and causing this ethylene oxide to react with water thereby obtaining ethylene glycol. In the production of ethylene glycol by the supply of the aqueous ethylene glycol solution to a concentrating treatment at the multi-effect evaporator, the method contemplated by US 2002/010378 for the production of ethylene glycol comprises utilizing as the source of heating at least one specific step the steam generated in the multi-effect evaporator.
U.S. Pat. No. 6,288,287 discloses a process for preparing a glycerol from a crude glycerol comprising a glycerol, a diol and water, comprising feeding the crude glycerol to a preparation apparatus comprising two or more, serially connected flash towers and a distillation tower connected to a final flash tower, wherein a bottom fraction of each flash tower is fed to a subsequent flash tower. According to U.S. Pat. No. 6,288,287, there is provided a process for preparing a glycerol from a crude glycerol comprising a glycerol, a diol and water, comprising the steps of: feeding the crude glycerol to a preparation apparatus comprising two or more, serially connected flash towers and a distillation tower connected to a final flash tower, wherein a bottom fraction of each flash tower is fed to a subsequent flash tower.
U.S. Pat. No. 5,269,933 and EP 0 532 905 A disclose a method for separation of a mixture of an organic fluid and water, such as carboxylic acids, including, for example, acetic acid, propionic acid, aromatic amines, including, for example, aniline, phenol, and glycerin. The method is a combination of a distillation, a water-selective pervaporation and a reverse osmosis and is particularly suitable for the separation of glycol and water. The process according to U.S. Pat. No. 5,269,933 comprises distilling the mixture, performing a water-selective pervaporation to bottom product obtained from the distillation step to obtain a residue and permeate, applying a reverse osmosis to at least the distillate to obtain a residue and permeate, and feeding the residue obtained from the reverse osmosis for performing thereon the distillation step, such that segregated organic fluid is present as residue from the water-selective pervaporation, and segregated water is present as the permeate from the water-selective pervaporation step as well as the permeate from the reverse osmosis step.
According to EP 0 324 915 A, valuable materials can be separated from aqueous solutions by extraction and/or distillation; the solution can be previously concentrated by reverse osmosis. The valuable materials are separated solely by distillation following concentration of the solution by optionally multi-stage reverse osmosis. According to the process disclosed in EP 0 324 915 A, the valuable material leaves the distillation column with the bottom stream or the top stream.
U.S. Pat. No. 6,712,882 discloses a process for treating waste water from an industrial process for producing propylene oxide, which process involves the steps of: (a) subjecting the waste water to a multi-effect evaporation treatment resulting in a vaporous top fraction and a liquid bottom fraction containing the non-volatile contaminants; and (b) condensing at least part of the vaporous top fraction into a liquid stream which is subject to a stripping treatment resulting in an overhead stream containing volatile waste organic material and purified water as the liquid bottom stream.
Accordingly, the prior art describes several options for treating waste water comprising organic compounds.
It is an object of the present invention to provide a process for recovering propylene glycol from a mixture comprising propylene glycol and water.
It is yet another object of the present invention to provide a process for recovering propylene glycol from a mixture comprising propylene glycol and water with improved yield and improved long term performance.
It is yet another object of the present invention to provide a process for recovering propylene glycol from a mixture comprising propylene glycol and water, wherein this mixture is obtained as a by-product from an epoxidation process where propene is oxidized to obtain propylene oxide.